Is there a way to do this "splitscreen" partially-rendered effect?
You may easily achieve this effect using Compositing Nodes. I'll show you how to do it both for a still image and animation.  


STILL IMAGE




First select the Slot 1 in the UV Editor window and render your scene using a Full render (F12).



Then change the slot to Slot 2, but this time render the active viewport using the Open GL render. Remove all the materials from your model. Check the Only Render checkbox, located in a Display panel of the Properties Shelf (N) and click the camera icon located at the bottom bar of the 3D window.



Save both images to your computer.


Now go to the Node Editor, press the Compositing nodes button and check the Use Nodes and Backdrop checkboxes. Then set up the nodes as pictured below. First add two Image nodes (Shift+A-->Input-->Image) and open both images you've rendered. Next create a simple b&amp;w texture in gimp (with the same resolution as rendered images).



Use it as a Factor for mixing two renders (add a Color node [Shift+A-->Color-->Mix] and plug the image to its Fac socket. Add a Viewer node (Shift+A-->Output-->Viewer) to see the compositing result.





ANIMATION




You may do it in similar way. Go to Render header and set up things as you like in Dimensions panel. In the Output panel choose the path to the folder you want your rendered frames to be saved (in my case I created a new folder and named it 'full render'). Change the file format to JPEG.


Now choose the Render Animation (Ctrl+F12).


When finished with rendering, change the path to another folder (I created a second one and called it 'openGL').


Press the OpenGL render animation button (the one next to OpenGL render button).


You have now two rendered image sequences in two folders. Set up the nodes as pictured below. Instead of adding Image nodes, add two Image Sequence ones (Shift+A-->Input-->Image Sequence). Load the rendered pictures for each of them.


With setup like this go once again to the Output panel, click on the file directory browser and create a new folder for the final animation. Change the file format to for example AVI Raw. Render your scene once again using F12 button.